It has long been believed that cellulosic and polymer materials sorb moisture in response to applied mechanical stress. W. W. Barkas introduced the concept of elasticity of a gel and calculated the equilibrium between external constraints and the swelling pressure. (Transactions of the Faraday Society 38 (1942)).
In 1962 Kubat and Nyborg of the Swedish Forest Products Laboratory reported that the moisture content of an unbleached kraft paperboard increased from 10.4% to 10.54% when the specimen was loaded to 50% of its tensile strength at a constant 65% relative humidity. They estimated the moisture content gain under load by observing the rate of moisture loss when the load was removed. This approach was necessary because it is not feasible to measure directly the extremely small weight changes which result from the strain in an apparatus which must develop large mechanical force. The Kubat method does not permit direct correlation of sorption with applied load, but did provide a first approximation of the magnitude of the strain-sorption effect. This method is complicated by the interaction of thermodynamic effects. A more sophisticated approach is clearly desirable. The present invention offers the potential for providing unique insight into bonding and functioning of the fiber network in paper and other materials while under stress.